| The purpose of this study was to examine the overtone spectrum of gaseous heterocyclic amines. The principal focus of this work centered on pyridine and the comparison of its spectrum to that of pyrrole. Pyridine should have three CH overtone progressions corresponding to the three different sets of CH bonds in the molecule. However, as in the liquid phase, only two peaks were observed at each overtone level. The overtone spectra of 3-fluoropyridine and 2,6-difluoropyridine were also recorded to confirm the CH overtone assignments and study the bond length-transition frequency relationship in pyridine. These spectra also exhibit two progressions of CH overtone peaks that can be assigned in the local mode model by comparison to pyridine and to ab initio CH bond lengths. These assignments are supported by trends in peak widths. The overtone spectra of 2,6-dichloropyridine, 3,5-dichloropyridine, and 3,4,5-trichloropyridine also support these assignments. From the pyridine and substituted pyridine spectral data, empirical expressions were derived for the relationship between CH bond length and overtone frequency. The aromatic heterocycles pyrazole and pyrimidines were also studied and compared to pyridine and pyrrole. Finally, the role of vibrational coupling of the CH bonds was investigated using the HCAO model. Different coupling schemes were examined to determine whether the experimental spectrum could be reproduced. The main deficiency of all the cases examined was the model's prediction of multiple weaker peaks to higher energy of the two main transitions. One area of this model that still needs to be explored is the calculation of peak intensities. Inclusion of intensities in the HCAO model derived for pyridine may yield results closer to the experimental spectra. |
